1. Field of the Invention
The present invention relates to a synchronizer clutch for a motor vehicle multistep transmission, having a guide sleeve which is fixedly arranged on a shaft, and a synchronizer ring which is mounted so as to be moveable relative to the guide sleeve.
2. Description of the Related Art
Synchronizer clutches of this type are used to connect a loose wheel, which is rotatably mounted on a shaft, to the shaft in a positive manner by means of shift gear toothing, and to disconnect this connection again.
Such arrangements are used in motor vehicle multistep transmissions which are embodied as countershaft transmissions. The motor vehicle multistep transmission in this case has a plurality of wheel sets which each comprise a fixed wheel and a loose wheel. The wheel sets are in constant rotational engagement. Different transmission ratios are established in the motor vehicle multistep transmission depending on which loose wheel of the various wheel sets is connected in a rotationally fixed manner to the assigned shaft.
Compared to simple positive shift clutches (such as claw clutches), synchronizer clutches have the distinctive feature that means are provided in order to match the rotational speeds of the loose wheel and the shaft to one another before the positive connection is generated. So-called locking synchronizer clutches are additionally set up such that the positive connection can only be established when the rotational speeds are substantially matched to one another.
Synchronizer clutches generally have a shift sleeve or sliding sleeve which is mounted on the guide sleeve so as to be axially displaceable and which has an inner shift gear toothing. A clutch body, which has an outer shift gear toothing, is normally fixedly arranged on the loose wheel. The positive connection is established by moving the sliding sleeve over the clutch body by virtue of the fact that the shift gear toothings engage in one another.
For the purposes of rotational speed synchronization, a conical friction face is provided on the clutch body. In a corresponding manner, a corresponding conical friction face is provided on the synchronizer ring which is mounted so as to be axially displaceable between the guide sleeve and the clutch body.
The synchronizer ring is moved axially away from the guide sleeve by displacing the sliding sleeve, so that the friction faces come into contact with one another and the rotational speeds are consequently matched.
In the case of locking synchronizer clutches, the synchronizer ring is provided with an outer locking gear toothing. In this case, the synchronizer ring is generally mounted so as to be rotatable to a limited extent with respect to the guide sleeve. Until the rotational speeds have been matched during a shift operation, the synchronizer ring is rotated relative to the inner shift gear toothing of the sliding sleeve on account of the frictional contact with the clutch body, so that it is not possible to shift through.
Only when the rotational speeds of the shaft and the loose wheel are at least substantially matched to one another can the synchronizer ring be rotated, by the shift force, back into a position in which it is possible to shift through.
When a shift operation is initiated, the synchronizer ring is centered on the friction cone of the clutch body and for this purpose moved away from the guide sleeve in the axial direction. The pre-synchronizing force required for this purpose is applied to the synchronizer ring by special pre-synchronizing devices. The magnitude of the pre-synchronizing force is limited. In the case of manual shift transmissions, the magnitude of the pre-synchronizing force is also a determining parameter in the comfort of the shift operation.
It has now been shown that when the transmission is cold, the shift force applied by the driver is not utilized completely for the build-up of the pre-synchronizing force which is required for applying the frictional torque to the conical friction faces. Above certain shift speeds, this inevitably leads to failure of the presynchronization and thus also to a failure of synchronization, which is represented by a crunching or scraping sound when shifting in a transmission which is not yet at operating temperature.